1. Technical Field
This invention relates to random orbital sanders, and more particularly to a random orbital sander having an annular member which acts as a brake on a platen of the sander to control the speed of the platen when the platen is lifted off of a work surface during operation of the sander.
2. Discussion
Random orbital sanders are used in a variety of applications where it is desirable to obtain an extremely smooth surface free of scratches and swirl marks. Such applications typically involve wood working applications such as furniture construction or vehicle body repair applications, just to name a few. In either of these applications it is imperative that the work surface be free of scratches and/or swirl marks before the surface is painted or otherwise coated with some form of finish sealer or stain.
Prior developed random orbital sanders typically include a platen which is driven rotationally by a motor-driven spindle within the sander. The platen is driven via a freely rotatable bearing that is eccentrically mounted on the end of the drive spindle. Rotation of the drive spindle causes the platen to orbit about the drive spindle while frictional forces within the bearing, as well as varying frictional loads on the sanding disc attached to the platen, cause the platen to also rotate about the eccentric bearing, thereby imparting the "random" orbital movement to the platen.
Typically such random orbit sanders also include a fan member which is driven by the output shaft of the motor. The fan member is adapted to draw dust and debris generated by the sanding action up through openings formed in the platen and into a filter or other like dust collecting receptacle. However, since at least some small degree of clearance must be left between the bottom housing or shroud of the sander and the upper surface of the platen to provide unimpeded rotational movement of the platen relative to the housing, this reduces the suction force that can be generated through the openings in the platen. Consequently, the ability of the fan to draw dust directly off of the work surface and through the openings in the platen is reduced.
However, a problem with most prior random orbit sanders is that the operator must be extremely careful not to gouge or mar the surface of the workpiece when the sander is applied to the workpiece. In particular, because the rotational speed of the platen varies with loading, when the platen is lifted off of the work surface the platen tends to spin up to an unloaded or "free wheeling" speed approximately equal to the rotational speed of the drive spindle, which may, for example, be as high as 12,000 rpm. Consequently, when the sanding disc is reapplied to the workpiece, the platen undergoes rapid deceleration, causing the sander to "jump" around on the workpiece until the rotational speed of the platen is reduced below approximately 1200 rpm. Accordingly, once the sander has been removed from the workpiece, it is difficult for the operator to gradually reapply the sander to the workpiece without the sander jumping and scratching the work surface.
Various attempts have been made to overcome this problem. For example, it has been proposed to provide a planetary gearing mechanism between the drive spindle and the platen to control the rotational speed of the platen. However, this approach adds complexity and is costly. Additionally, it has been proposed to include a spring-mounted braking member that is biased against the platen to frictionally control the speed of the platen. However, such devices, while also adding cost, tend to cause excessive wear of the platen and can have the undesirable effect of reducing the rotational speed of the platen below its minimum effective operating speed when loaded. Moreover, frictional wear can also reduce the axial dimension of the braking component significantly over many hours of use which, in turn, reduces the spring force, or braking force, which the component can exert on the platen. Accordingly, such components have not been able to provide the relatively constant braking force needed, over many hours of use, to provide consistent speed control over the platen. Exacerbating this problem is the varying types of abrasive particulate matter which the sander is inevitably exposed to. This further accelerates the wear of the braking component and platen and contributes to the degradation in the braking force of such frictional braking components.
Thus, it is highly desirable to control the speed of the platen when the platen is lifted off of a work surface while allowing the platen to rotate at a speed of at least about 800 rpm when being applied to the work surface. Controlling the speed of the platen by preventing it from exceeding about 1200 rpm allows the sander to be lifted off of and reapplied to the work surface without marring the work surface and thus enhancing the sanding operation. It is further highly desirable to provide this control via a single, relatively inexpensive frictional braking component which is resistant to frictional wear.
It is therefore a principal object of the present invention to provide a brake member for a random orbit sander which serves to allow the platen of such a sander to rotate at a speed of at least about 800 rpm while the platen is being applied to a work surface, and yet which serves to provide a frictional "braking" action to the platen when the platen is lifted off of the work surface so as to limit the rotational speed of the platen to a predetermined upper limit.
It is yet another object of the present invention to provide a braking member for a random orbit sander which serves to effectively seal the small distance between the upper surface of the platen and the lower surface of the housing to thus cause the full suction force generated by the fan to draw sanding dust up through openings in the platen, to thereby remove such dust directly from the work surface being sanded.
It is still another object of the present invention to provide a braking member for a random orbit sander which serves to provide a relatively constant braking force to a platen of the sander, and which does not wear appreciably due to frictional contact with the platen, after an initial break-in period.
It is still another object of the present invention to provide a braking member for a random orbit sander which provides a relatively constant braking force to the platen which is generally unaffected by the form of sanding dust from the work surface being sanded.
It is still another object of the present invention to provide a braking member for a random orbit sander which may be relatively easily retrofitted to many existing sanders without major structural modifications to the braking member itself or to the housing of the sander.